The design of chemically amplified resist systems based on acid-catalyzed chain reactions (e.g., polymerization, depolymerization, side chain cleavage, etc.) has been recognized as a viable route to high sensitivity resist systems in microlithography. Such systems are described in, e.g., Polymers In Electronics, Davidson T. Ed., ACS Symposium Series 242, American Chemical Society, Washington, D.C., 1984, page 11, H. Ito, C. G. Willson. These systems offer the high sensitivity attendant with chemical amplification but avoid the drawbacks of free radical based systems. Systems of this type are capable of high resolution, i.e., submicron in the case of semiconductor applications. Since they are oxygen insensitive, they can be applied in liquid form as thin films.
Photoresists which function via an acid-catalyzed deprotection mechanism utilize polymers which are positive acting. That is, the unexposed resist is insoluble in the developer, but is converted into a soluble material upon exposure. Chemically, this is accomplished by changing the side group chemistry through thermal cleavage, a reaction which is acid-catalyzed. Compounds such as iodonium salts generate acid upon photo or thermally induced decomposition. These materials are typically sensitized to the desired wavelengths with an appropriate dye.
However, the use of iodonium salts and non-ionic triflic acid generators in positive acting photoresists on copper surfaces has been impractical due to the formation of an undevelopable layer of resist at the copper surface in the exposed regions. Additionally, photoresists have often proved difficult to strip in standard basic aqueous solutions such as sodium hydroxide. Attempts to overcome these problems have included copper passivation methods, such as electroless chrome or nickel treatments, or the use of alternative photoacid generators, such as sulfonium salts. Unfortunately, the copper passivation methods were impractical since they involved additional process steps and often utilized toxic materials. Similarly, sulfonium salts did not perform well because they are difficult to sensitize to visible light resulting in low photospeeds.